iconvdata: Split out non-essential gconv module configuration

Split module configuration so that only the bare minimum charsets,
i.e. ANSI_X3.110, ISO8859-15, ISO8859-1, CP1252, UNICODE, UTF-16,
UTF-32 and UTF-7 are configured in gconv-modules.conf.  The remaining
module configurations are now in gconv-modules-extra.conf.

Reviewed-by: DJ Delorie <dj@redhat.com>